The prior art comprises:
i) a so-called "non integral protection", or open protection, in which a single casing body protects the perimeter of the joint, but leaves an opening through which a foreign body may reach rotating parts (see for example the European Patent n.degree. 0 154 216 held by the same applicant). If one considers that the foreign body might be a person's hand, it is clear why such a protection is dangerous. This form of protection is fitted into a perimetral groove of the disc between the two yokes of the coupling where it is held in position by chainlets, and is a conical piece with the greater aperture facing the transmission shaft so that there are no problems when the shaft inclines, for example when the tractor goes round a bend.
ii) a so-called "semi-closed petal protection" comprising a main casing that covers the whole joint around the perimeter and an auxiliary casing that covers the end of the transmission shaft that connects to the joint. The space between the two casings, set at a certain distance from one another, allows relative movements between the same casings when the transmission shaft inclines and is closed by a series of plastic "petals" that have their external margins fastened within the main casing and their internal margins loose, but touching the auxiliary casing. This solution offers a certain amount of protection and the possibility of high inclinations of the transmission shaft, but has its main drawback in the fact that, because the "petals" are elastic, they may bend even under a weak pressure allowing a foreign body to reach the rotating parts of the transmission means.